1. Field of the Invention
The present invention relates in general to a communication system, and more particularly to improvements of an interrogator and endpoint or local devices (e.g., so-called “tags”) included in a communication system which is arranged such that the endpoint devices which have received an interrogating signal transmitted from the interrogator respond to the interrogator with modulated reflected signals.
2. Discussion of Related Art
There is known a communication system wherein an interrogator transmits a main carrier to a plurality of endpoint devices or local devices (which may be so-called “tags”), and each endpoint device which has received the main carrier responds to the interrogator with a reflected signal which is generated by modulating the main carrier with appropriate information such as an identification signal which identifies the endpoint device. In this type of communication system, the endpoint devices are available at a comparatively low cost, so that the system has a high degree of cost performance, particularly where the system includes a relatively large number of endpoint devices. On the other hand, however, this communication system tends to suffer from a problem of a collision among the reflected signals transmitted from a large number of endpoint devices. In an attempt to solve this drawback, there have been proposed some solutions, such as 1) provision of a time slotted structure including a plurality of time slots or frames for the respective endpoint devices to respond to the interrogator, and 2) frequency hopping technology involving frequency division multiplexing of subcarrier signals used for modulating reflected signals to be transmitted from the endpoint devices, and random selection of the subcarrier signal frequency for each reflected signal. Examples of the above-indicated solutions are disclosed in U.S. Pat. No. 6,177,861 B1 (corresponding to JP-2000-49656 A).
However, the provision of the plurality of time slots requires a long time to complete the data communication of the interrogator with all of a large number of endpoint devices.
On the other hand, the frequency division multiplexing technology using the frequency hopping reduces the time required to complete the data communication of the interrogator with all of the endpoint devices. However, the reflected signal transmitted from an endpoint device which is relatively distant from the interrogator may interfere with the reflected signals transmitted from endpoint devices relatively near the interrogator, or with higher harmonics of the reflected signals. In this case, the interrogator may not even recognize the presence of the reflected signal transmitted from the endpoint device distant from the interrogator. Where the plurality of endpoint devices transmit the reflected signals using the same subcarrier frequency, the interrogator may not correctly demodulate the reflected signals if the reflected signals have substantially the same intensity. In this event, the interrogator can transmit a retry request signal asking the endpoint devices to repeat the transmission of the reflected signals. When the reflected signal transmitted from the endpoint device relatively distant from the interrogator interferes with the reflected signals transmitted from the endpoint devices relatively near the interrogator or the higher harmonics of the reflected signals, the interrogator can correctly demodulate the reflected signals from the nearby endpoint devices, so that the interrogator does not transmit the retry request signal, resulting in a failure of the interrogator to recognize the reflected signal transmitted from the relatively distant endpoint device.
In a communication system of the frequency hopping type wherein the number of endpoint devices varies, the frequency range in which the subcarrier frequency hopping is implemented need to be as broad as possible, in order to reliably avoid a collision among the reflected signals from the endpoint devices. However, an increase in the subcarrier frequency range results in an increase in the ratio of the used subcarrier signals having relatively high frequencies, leading to an accordingly increased amount of consumption of electric power by the endpoint devices, and an accordingly reduced service life of the endpoint devices where the endpoint devices have a relatively small electric energy storage capacity.
U.S. Pat. No. 6,177,861 B1 also discloses a technique to change the timing for the endpoint devices to respond to the interrogator, for the purpose of avoiding a collision among the reflected signals. However, this technique requires a relatively long data communication time due to the time required for changing the timing. To overcome this drawback, it is considered to use a large number of spread codes for establishing a quasi-random system, in order to avoid the collision. This technique requires a complex operation to assign the spread codes to the respective endpoint devices, and requires the interrogator to discriminate the large number of spread codes from each other.